This invention relates to a device and method for detecting the position of a portable compact wireless terminal. More particularly, the invention relates to such a device and a method for making a detection within a limited range of area such that when the invention is applied to the keyless entry system of a vehicle such as an automobile, for example, it becomes possible to detect whether the terminal of the system is inside the vehicle or not and enables the user to control the vehicle such as the unlocking of its door, corresponding to the result of its position detection and to carry out a control of the vehicle corresponding to the result of this position detection.
In recent years, some vehicles such as automobiles (hereinafter simply referred to as “vehicles”) are coming to be produced with a convenient system mounted thereto for allowing the door to be unlocked or the engine to be started without using a mechanical key but merely by installing a small wireless terminal (hereinafter referred to as a “portable device”) near or inside the chamber of the vehicle. Such systems carry various different names such as “smart entry system” and “intelligent key system”.
The component of such a system carried inside the vehicle must be always aware whether its portable device is inside the chamber of the vehicle or not. It is because it will be like locking in a car key if the request button for the door is pressed with the portable device left inside the vehicle chamber. For such a situation, the system must be designed so as to output an alarm or not to lock the doors. It should thus be clear that such a position detector is necessary also from the point of view of crime prevention. What would happen if a total stranger pressed the request button while the vehicle is temporarily stopped, say, by a red light at a cross-road? For such a situation, the operation on the request button should be invalidated so as to prevent the total stranger from invasively entering the vehicle.
Japanese Patent Publication Tokkai 2003-3715 disclosed a technology of detecting the position of such a portable device (that is, whether it is inside the vehicle chamber or not), providing a (first) transmission antenna with the transmission area limited to the exterior of the vehicle and another (second) transmission antenna with the transmission area limited to the interior of the vehicle. According to this technology, request signals are transmitted to the portable device at different timings from the two antennas such that the position of the portable device can be detected from the presence or absence of an answer signal returned in response to these request signals. If the answer signal is returned when the request signal is transmitted from the first antenna, it is concluded that the portable device is outside the vehicle chamber. If the answer signal is returned when the request signal is transmitted from the second antenna, it is concluded that the portable device is inside the vehicle chamber. This technology is disadvantageous because two transmission antennas are required and hence this affects the production cost adversely.